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AP Bio Ch 16 part 2
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AP Bio Ch 16 part 2


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  • 1. Proteins Work Together in DNA Replication & Repair Franklin Stahl
  • 2. The Basic Principle: Base Pairing to a Template Strand Watson & Crick’s hypothesis = a Semi-Conservative model Half of each new molecule is comprised of a original template strand (dark blue) A T A T C G C G C T A T A T A G T C A G T C T A A G G A T A T A T A T C G C G C G T A T A T A A C T C T A T G C G C A G Original DNA Segment Hydrogen bonds _____?______ that hold the 2 strands together are temporarily broken The two new Each original segments are strand serves as a identical to each template for a new, other AND identical to the original. complementary strand
  • 3. Quick Think How DOES DNA’s structure allow for it’s exact replication???
  • 4. Although Watson & Crick Proposed a SemiConservative Model, Other Alternatives Were Proposed
  • 5. Which Model Does DNA Replication Actually Follow? The Meselson-Stahl Experiment:
  • 6. Quick Think Describe what semiconservative replication means
  • 7. DNA Replication: Starting DNA replication begins at a special sequence of nucleotides which signifies an origin of replication-> proteins pull the two strands apart to form a replication “bubble.” Replication proceeds in both directions:
  • 8. DNA Replication: Elongating the •DNA polymerases New Strand catalyze the addition of a nucleoside triphosphate to the 3’ end of a growing DNA strand •A nucleoside triphosphate is a nucleotide monomer with 3 P groups the 3 P groups create enough instability that 2 P are lost during the addition of the monomer. •Hydrolysis of the 2 P molecule, pyrophosphate, releases energy that drives the polymerization reaction
  • 9. DNA Replication: Antiparallel Elongation •The 2 strands of a DNA molecule are antiparallel to each other; they’re oriented in opposite directions •DNA polymerases add nucleotides only to the 3’ end of a growing chain. Therefore growth proceeds in the 5’3’ direction
  • 10. The leading strand is made continuously & in one piece Okazaki fragments are then joined together by DNA ligase The lagging strand is made small chunks, Okazaki fragments, in order to follow the 5’3’ rule
  • 11. Leading vs. Lagging Strand
  • 12. Priming DNA Synthesis •DNA replication cannot begin without a primer •A primer is a short segment of RNA that has an available 3’ end •Primase is a special enzyme that constructs the primer sequence from scratch •The leading strand only needs one primer. However the lagging strand needs several primers •DNA pol I replaces RNA primer nucleotides with DNA after each fragment is made •Ligase then fuses fragments
  • 13. Quick Think Describe the role of primase, DNA pol I, DNA pol III, and ligase in the formation of the lagging strand.
  • 14. Summary of Bacterial DNA Replication Helicase unwinds the helix at the replication fork Single-strand binding protein stabilizes single stranded DNA until it can be used as a template Topoisomerase keeps DNA from overwinding by breaking & rejoining the DNA ahead of the replication fork
  • 15. Misconceptions & the Replication Machine Simplified models make it seem like the DNA molecule is stationary and that the various replication enzymes move down the molecule. Contrary to this idea, there is evidence that supports the reverse notion: Replication enzymes form a complex, a “machine,” and the DNA moves through the machine. It’s analogous to fabric moving through a sewing machine.
  • 16. Proof Reading & Repairing DNA •DNA polymerases are the molecules that “proof-read” each nucleotide to ensure that it is complementary to the template •If a mistake is found, the nucleotide is removed & replaced •If DNA polymerase doesn’t catch a mistake, or if a mismatch occurs after DNA replication, then excision repair is one way to correct the error: •A nuclease is a special enzyme that cuts DNA during repair
  • 17. Quick Think •What are some ways in which DNA may become damaged? •Why is it important for DNA to be able to repair itself?
  • 18. Replicating the Ends of DNA Molecules In linear DNA, such as eukaryotic DNA, there comes a point toward the end of the DNA molecule where a 3’ end is not available, and, therefore, DNA pol III can no longer add nucleotides: With each replication, the new DNA molecule is becoming shorter & shorter!!!
  • 19. To mitigate the effects of the progressive shortening, the ends of a DNA molecule are composed of telomeresrepeating sequences of DNA (in humans, TTAGGG) that don’t code for any genes. The presence of telomeres postpones the erosion of the end of a DNA molecule. Cells that undergo many divisions, like germ cells which produce gametes, contain an enzyme called telomerase. Telomerase catalyzes the lengthening of telomeres so that a zygote will contain cells that have maximum telomere length Bright yellow=telomeres
  • 20. Quick Think How are telomeres important in preserving eukaryotic genes? Why does prokaryotic DNA lack telomeres?